As already mentioned, take advice from a legal professional.
but it sounds to me like she has obtained her 'evidence' illegally?
Whatever it was she thinks this bag contains is now irrelevant as I would imagine it could not be proven that that she did or did not put it there herself (unless the guy who was sacked admitted to it, which I hope he has not).
True, all companies will take disciplinary action over drugs and alcohol, but managers have to stay on the correct side of the line themselves, otherwise they may well find themselves sacked.
This manager by her actions:
Cannot prove she did not plant the material in his belongings.
Cannot prove what the material is without having it tested in a lab.
Cannot prove that he was under the influence of drugs/alcohol at the time of the search unless they have the means to test him on the spot.
Cannot justify searching his belongings in the first place if there is no search policy present withing the company rules, let alone searching them in the manner she has done.
If this guy stood his ground I think the manager could be in a lot of trouble. If it is a family run business, and she is a member of the family, she probably wont get the sack because they will pull rank, however if she does not get sacked then it sets a precedence to all other employees and the company may find it hard to sack anyone ever again without being hauled into a tribunal of some sort.